A photo of an empty syringe stuck into a model of the coronavirus, which is made out of a styrofoam ball painted purple, with multicoloured quilter's pins sticking out if it, with a white background.
Photo: Ivan Diaz/Unsplash

An analysis of data from 21 studies suggests that more than 17 million Canadians were infected with the Omicron variant in just five months.

“Omicron has been a tsunami,” COVID-19 Immunity Task Force (CITF) executive director Dr. Tim Evans said in a media release Wednesday morning.

“Across the country, our analysis of the data suggests that 17 million Canadians had an Omicron infection in the period December to May, for an average of more than 100,000 infections per day.”

Evans said because new sublineages in the Omicron line have continued to spread since then, the percentage of Canadians who have had a SARS-CoV-2 infection is now likely “well above 50%.”

The data analysis of 21 studies was funded by the federal government through CITF and relied on blood testing. The results were published in a new CITF report titled ‘Seroprevalence against SARS-CoV-2 due to infection in Canada’ and included data received up to May 31.

Before Omicron’s arrival, about 7% of Canadians had infection-acquired antibodies to the virus that causes COVID-19. Between December 2021 (when the Omicron wave emerged) and May 2022, that number climbed by 45%.

Atlantic Canada had largest relative increase

The report found the number of people with signs of a previous infection in their blood (infection-acquired seroprevalence) increased significantly during the Omicron wave in every province.

By the end of May, the proportion of people with evidence of previous infection was 50% to 60% in the Western and Central provinces.

“Although Atlantic Canada retained the lowest seropositivity due to infection, it had the largest relative increase in seroprevalence and reached over 35%,” the release noted.

Chart showing the estimates of infection-acquired seropositivity by province.
From the new COVID-19 Immunity Task Force report.

Younger, less vaccinated most affected

While Omicron affected all ages, researchers found that younger and less vaccinated populations were the most impacted. Analysis of blood donations made to Canadian Blood Services and shared with CITF showed highest seroprevalence due to infection was in young adults —about 65% had antibodies by the last week of May.

The older the population group, the more the rates of seropositivity due to infection dropped. In Canadians 25 to 39, it was 57%. For those in the 40 to 59 age group, it was 51%. In the population of Canadians 60 and older, it was 31%.

“Through sheer numbers of infections, the Omicron variant exacted a substantial toll in services and lives disrupted, as well as hospitalizations and deaths. It clearly did not spare healthy young Canadians” CITF Co-Chair Dr. Catherine Hankins said in the news release.

“As well, we’re still learning about who gets a post COVID-19 condition or long COVID, why, and the repercussions. This summer may be free of public health restrictions, but Omicron is still spreading so masking and distancing are smart in risky settings. To minimize further disruptions to our lives, Canada has to track the situation closely as it evolves. We all need to respond in a timely way as this virus does not have a seasonal pattern, like the other respiratory viruses we expect when everyone heads back to work or school in the autumn.”

Most are at risk of re-infection

Task force co-chair Dr. David Naylor said while millions of Canadians now have hybrid immunity from being vaccinated and having acquired a COVID-19 infection, emerging evidence suggests most remain at risk of re-infection with viruses in the Omicron lineage.

“Newer vaccines may improve coverage against infection. However, we still have millions of adults who haven’t had a third shot and should get one to consolidate their protection against serious disease,” Naylor said.

“More generally, all Canadians should stay alert for the latest public health guidance on COVID-19 vaccines and make sure their coverage is up to date for the fall season.”

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Yvette d’Entremont is a bilingual (English/French) journalist and editor who enjoys covering health, science, research, and education.

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